Field
Aspects of the present invention generally relate to a measurement apparatus for measuring the shape of an object (an object to be measured), a system, and a method for producing an article.
Description of the Related Art
As one of techniques of measuring the shape of an object, there is known an optical measurement apparatus. The optical measurement apparatus can employ various methods, which include a method called a “pattern projection method”. The pattern projection method is to find the shape of an object by projecting a predetermined pattern onto the object, capturing an image of the object having the predetermined pattern projected thereon to obtain a captured image, detecting a pattern in the captured image, and calculating distance information (range information) in each pixel position according to the principle of triangulation.
Such a measurement method detects coordinates of each line of the projected pattern based on spatial distribution information about the amounts of received light (pixel values) in the captured image. However, the spatial distribution information about the amounts of received light is data including influences of, for example, a reflectance distribution caused by a pattern or design of the surface of an object to be measured or a reflectance distribution caused by a fine-structure shape of the surface of an object to be measured. These influences may cause detection errors in detecting the coordinates of a pattern or may make it impossible to perform such detection itself, and, as a result, information on the calculated shape of an object to be measured may become less accurate.
On the other hand, a measurement method discussed in Japanese Patent Application Laid-Open No. 3-289505 projects pattern light onto an object to be measured using a liquid crystal shutter and acquires a captured image of the object having the pattern light projected thereon (hereinafter referred to as a “pattern projection image”), and then irradiates the object with uniform illumination light using the liquid crystal shutter and acquires a captured image of the object irradiated with the uniform illumination light (hereinafter referred to as a “uniform illumination image”). Then, the measurement method performs correction to remove, from the pattern projection image, an influence caused by a reflectance distribution of the surface of the object using data of the uniform illumination image as corrective data.
Furthermore, a measurement method discussed in Japanese Patent Application Laid-Open No. 2002-213931 irradiates an object with both pattern light and uniform illumination light which have respective polarization directions differing 90 degrees from each other and acquires a pattern projection image and a uniform illumination image using respective imaging units corresponding to the respective polarization directions, and then performs image processing to obtain distance information (range information) from a difference image between the pattern projection image and the uniform illumination image. This measurement method acquires the pattern projection image and the uniform illumination image at the same timing and performs correction to remove, from the pattern projection image, an influence caused by a reflectance distribution of the surface of the object.
The measurement method discussed in Japanese Patent Application Laid-Open No. 3-289505 acquires the pattern projection image and the uniform illumination image at different timings. In view of the intended use of a measurement apparatus, such a case can be seen where one of or both of an object to be measured and an imaging unit of the measurement apparatus are moving at the time of acquiring distance information. In this case, the relative positional relationship between them changes at the respective timings, so that the pattern projection image and the uniform illumination image become images captured from different points of view. In that case, if correction is performed using the images captured from different points of view, some errors may occur.
The measurement method discussed in Japanese Patent Application Laid-Open No. 2002-213931 uses two pieces of polarized light having respective polarization directions differing 90 degrees from each other, so that the pattern projection image and the uniform illumination image are acquired at the same timing. However, the surface of an object to be measured may have a local variation in angle due to a fine-structure shape (surface roughness) thereof, and such a local variation in angle may cause the reflectance distribution of the surface of the object to vary with polarization directions. This is because the reflectance of incident light with respect to the angle of incidence varies depending on polarization directions. Accordingly, if correction is performed using images containing information on mutually different reflectance distributions, some errors may occur.